Leaf waxes from Lathyrus sativus: short-range attractant and stimulant for nymph laying in a viviparous insect

Epicuticular wax chemicals of Lablab purpureus subsp. bengalensis influence short-range attraction and oviposition responses in Aphis craccivora and Aphis gossypii

Lablab purpureus subsp. bengalensis (Jacq.) Verdc. is an important legume of India and Africa. Both aphids, Aphis craccivora Koch and A. gossypii Glover (Hemiptera: Aphididae), are important herbivorous pests of this legume crop. These viviparous females lay nymphs on the leaf surface of this legume plant. Therefore, it is of considerable interest to study whether leaf surface wax chemicals (long-chain alkanes and free fatty acids) of this legume plant served as short-range attractants and oviposition stimulants in both females to lay nymphs. Twenty-one n-alkanes from n-C12 to n-C35 and 11 free fatty acids from C12:0 to C22:0 were identified in leaf surface waxes. Nonacosane and nonadecanoic acid were the most abundant among n-alkanes and free fatty acids, respectively. Both females were attracted towards one leaf equivalent surface wax against the control solvent (petroleum ether) in short Y-tube olfactometer bioassays. A synthetic blend of tetradecane, pentadecane, tetracosane, tridecanoic acid, tetradecanoic acid, and heneicosanoic acid comparable to one leaf equivalent surface wax served as short-range attractants and oviposition stimulants in A. craccivora; whereas a synthetic blend of tetradecane, hexadecane, docosane, nonadecanoic acid, and arachidic acid comparable to one leaf equivalent surface wax acted as short-range attractants and oviposition stimulants in A. gossypii. These results can provide the basis for efficient pest management strategies of A. craccivora and A. gossypii against L. purpureus subsp. bengalensis using host plant leaf surface wax compounds. Further, SEM studies of antennae and forelegs of both aphids were conducted to observe sensilla structures, which help in chemoreception.

Elevated ozone alters long-chain fatty acids in leaves of Japanese white birch saplings

Long-chain fatty acids (LCFAs) in leaves have attracted attention as nutritious phytochemicals and olfactory signals that influence the behavior and growth of herbivorous insects. In recognition of the negative effects of increasing tropospheric ozone (O₃) levels on plants, LCFAs can be altered through peroxidation by O₃. However, how elevated O₃ changes the amount and composition of LCFAs in field-grown plants is still unknown. We investigated palmitic, stearic, oleic, linoleic, linolenic LCFAs in the two leaf types (spring and summer) and two stages (early and late stage after expansion) of Japanese white birch (Betula platyphylla var. japonica) after a multi-year O₃ exposure on the field. Summer leaves exhibited a distinct composition of LCFAs under elevated O₃ at the early stage, whereas both stages of spring leaves did not exhibit significant changes in LCFAs composition by elevated O₃. In the spring leaves, the amounts of saturated LCFAs significantly increased at the early stage, however, the amount of total, palmitic, and linoleic acids at the late stage were significantly decreased by elevated O₃. Summer leaves had a lower amount of all LCFAs at both leaf stages. Regarding the early stage of summer leaves, the lower amount of LCFAs under elevated O₃ was possibly due to O₃-suppressed photosynthesis in the current spring leaves. Furthermore, the decrease ratio of spring leaves over time was significantly increased by elevated O₃ in all LCFAs, whereas summer leaves did not exhibit such an effect. These findings suggest that further studies should be conducted to reveal the biological functions of LCFAs under elevated O₃, considering the leaf type- and stage-dependent changes of LCFAs.

Short-range attraction and oviposition stimulant of a biocontrol agent, Galerucella placida Baly (Coleoptera: Chrysomelidae) toward weed leaf surface waxes

Two Polygonaceae weeds, Rumex dentatus L. and Polygonum glabrum Willd. are abundant in wheat- and rice-fields, respectively, in India. Galerucella placida Baly (Coleoptera: Chrysomelidae) is a biocontrol agent of these two weeds. The importance of long-chain alkanes and free fatty acids present in leaf surface waxes of these weeds was assessed as short-range attractant and ovipositional stimulant in G. placida females. Extraction, TLC, GC-MS and GC-FID analyses demonstrated 19 n-alkanes from n-C14 to n-C35 and 14 free fatty acids from C12:0 to C22:0 in leaf surface waxes. Hentriacontane was predominant among alkanes in both weeds, while oleic acid and docosanoic acid were predominant among free fatty acids in R. dentatus and P. glabrum, respectively. Females of G. placida were attracted toward one leaf equivalent surface wax of both weeds against the control solvent (petroleum ether) in a short Y-tube olfactometer bioassay. But, the insect could not differentiate between one leaf equivalent surface wax of R. dentatus and P. glabrum, indicating that both weed leaves were equally attractive in females. A synthetic blend of either 2.44, 35.57 and 23.58 μg ml-1 of octadecane, heptacosane and nonacosane, respectively, resembling the amounts present in one leaf equivalent surface wax of R. dentatus or 4.08, 19.54 and 23.58 μg ml-1 of octadecane, palmitoleic acid and docosanoic acid, respectively, resembling the amounts present in one leaf equivalent surface wax of P. glabrum acted as short-range attractant and ovipositional stimulant in G. placida. These results could be a basis for host plant specificity of the biocontrol agent.

Attraction of the Biocontrol Agent, Galerucella placida Towards Volatile Blends of Two Polygonaceae Weeds, Rumex dentatus and Polygonum glabrum

The Polygonaceae weed, Rumex dentatus L. grows in association with wheat, mustard and potato, while Polygonum glabrum Willd. grows in association with rice in India. Both larvae and adults of Galerucella placida Baly (Coleoptera: Chrysomelidae) voraciously consume these weeds. Applications of synthetic herbicides to control weeds are harmful to the environment including beneficial organisms. We propose to find volatile organic compounds (VOCs) from both weeds causing attraction of the biocontrol agent, G. placida, in order to attempt to use the insect as a biological weed control. Behavioral responses of G. placida towards volatile blends characteristic of undamaged (UD), insect-damaged (ID), jasmonic acid-treated (JA) or mechanically-damaged (MD) plants were conducted by Y-tube olfactometer bioassays. Cuminaldehyde was predominant in VOCs of UD R. dentatus, ID P. glabrum, and both JA and MD R. dentatus and P. glabrum. Geraniol was predominant in VOCs of UD P. glabrum, while 1,3-diethylbenzene predominated in VOCs of ID R. dentatus. Females were more attracted towards volatile blends of ID plants compared to UD or JA plants. Females did not show attraction towards volatile blends of JA plants. We identified two bioactive synthetics blends, one comprised of seven compounds - 16.65 µg 1,3-diethylbenzene, 10.72 µg acetophenone, 6.52 µg 2,6-(E,Z)-nonadienal, 2.46 µg 1-nonanol, 4.19 µg decanal, 9.86 µg 4-ethylacetophenone and 3.34 µg 1-hexadecene dissolved in 25 µl CH₂Cl₂ and the other containing five compounds - 2.50 µg 2-octanol, 6.84 µg limonene, 0.64 µg dodecane, 6.63 µg 4-ethylacetophenone and 0.24 µg geranyl acetone dissolved in 25 µl CH₂Cl₂. These two blends of volatile compounds could be used to attract the biocontrol agent during early vegetative period of these two weeds, which could lead to eradication of weeds from crop fields.

Synergism in Host Selection Behavior of Three Generalist Insects Towards Leaf Cuticular Wax of Sesame Cultivars

Leaf cuticular wax plays important role in host selection, oviposition, and feeding of phytophagous insects. Thus, the role of cuticular wax of sesame (Sesamum indicum) cultivars (Savitri and Nirmala) in host selection of 3 generalist pests (Spilosoma obliqua Walker, Helicoverpa armigera Hübner, and Spodoptera litura Fabricius) was investigated under laboratory conditions. The GC-MS and GC-FID analyses of leaf surface waxes of both cultivars indicated the presence of 14 n-alkanes from n-C₉ to n-C₄₄ and 12 free fatty acids (FFAs) from C_9:0 to C_20:0. The most predominant n-alkane and FFA of the cultivars were n-C₂₆ (94.3 ± 7.27 μg leaf^-1) and C_18:1 (110.8 ± 10.07 μg leaf^-1), respectively present in Savitri cultivar. The generalists used visual (color and shape), olfactory (odorous n-alkanes and FFAs), tactile (surface ultra-structure), and gustatory (cuticular wax) cues in a synergistic manner for their host selection through attraction (adults and larvae) followed by oviposition (adults) and feeding (larvae) on studied cultivars (Savitri > Nirmala). Their olfactory responses were maximum towards 2 leaf equivalent amount, whereas oviposition and feeding preference were maximum towards 4 leaf equivalent amount of the combined synthetic (4 n-alkanes (n-C₁₆, n-C₂₂, n-C₂₄, n-C₂₆) + 3 FFAs (C_12:0, C_14:0, C_18:1)) mixture-treated intact leaf of cultivar Savitri. This finding can suggest that the synthetic blend (4 n-alkanes + 3 FFAs) in leaf equivalent amount (396.6 ± 4.13 μg leaf^-1) or more from cultivar Savitri can be used as lures to develop baited trap. In addition, the cultivar Nirmala can be used as a resistant cultivar in the ecological pest management (EPM) framework of these target pest species.

Leaf Surface Wax Chemicals in Trichosanthes anguina (Cucurbitaceae) Cultivars Mediating Short-Range Attraction and Oviposition in Diaphania indica

Larval Diaphania indica (Saunders) (Lepidoptera: Crambidae) cause complete defoliation of Trichosanthes anguina L. and reduce crop yield in India. Females lay eggs on the leaf surface, and therefore leaf surface waxes are potentially involved in host selection. Alkanes and free fatty acids are the major constituents of leaf surface waxes, so a study was conducted to determine whether these wax constituents from three T. anguina cultivars (MNSR-1, Baruipur Long, and Polo No.1) could act as short-range attractants and oviposition stimulants in D. indica females. Twenty n-alkanes from n-C₁₄ to n-C₃₆ and 13 free fatty acids from C12:0 to C21:0 were detected in the leaf surface waxes of these cultivars. Heptadecane and stearic acid were predominant among n-alkanes and free fatty acids, respectively, in these cultivars. Females showed attraction towards one leaf equivalent surface wax of each of these cultivars against solvent controls (petroleum ether) in Y-tube olfactometer bioassays. A synthetic blend of heptadecane, eicosane, hexacosane, and stearic acid, a synthetic blend of hexacosane and stearic acid, and a synthetic blend of pentadecane and stearic acid comparable to amounts present in one leaf equivalent surface wax of MNSR-1, Baruipur Long, and Polo No.1, respectively, were short-range attractants and oviposition stimulants in D. indica. Female egg laying responses were similar to each of these blends, providing information that could be used to developing baited traps in integrated pest management (IPM) programs.

Identification of Lathyrus sativus plant volatiles causing behavioral preference of Aphis craccivora

BACKGROUND

The viviparous aphid Aphis craccivora Koch (Hemiptera: Aphididae) is a serious threat to the crop yield of Lathyrus sativus L. (Fabaceae), commonly known as grass pea. The synthetic insecticides applied to control this insect pest are not safe for the environment. Hence, it is necessary to find volatile organic compounds (VOCs) from two cultivars [BIO L 212 Ratan (BIO) and Nirmal B-1 (NIR)] of L. sativus plants causing behavioral preference of A. craccivora.

RESULTS

The VOCs from undamaged (UD), insect-damaged (ID) [plants on which 50 or 100 adults of A. craccivora were fed for 4 h (ID 50 or ID 100)], and mechanically damaged (MD) plants were identified and quantified by gas chromatography-mass spectrometry and gas chromatography-flame ionization detection analyses, respectively. Total VOCs were higher in ID plants compared to UD plants of each cultivar. However, total VOCs were higher in NIR cultivar compared to BIO cultivar for both UD and ID plants. Benzyl alcohol was predominant in volatile extracts of all treatments. In Y-tube olfactometer bioassays, females showed preference towards volatile extracts of UD, ID, and MD plants of each cultivar compared to the control solvent (CH₂ Cl₂ ). Insects preferred certain synthetic blends comparable to volatile extracts of UD, ID, and MD plants of each L. sativus cultivar against the control solvent.

CONCLUSION

Females preferred a synthetic blend of benzyl alcohol, 1,3-diethylbenzene, thymol, and 1-hexadecene at ratios of 142.49: 62.03:1.18:1 dissolved in 25 μL of CH₂ Cl₂ in olfactometer bioassays, which could be used in developing lures to control this insect pest.